It is often desirable, and in some instances even critical, for transistors to withstand exposure to radiation. Numerous military applications require transistors "hard" to a specified minimum dose of radiation. Failure of the transistor under radiation exposure can often have devastating consequences.
For mesa-isolated SOI (silicon-on-insulation) transistors, the effects of a parasitic transistor which forms along the mesa sidewalls can render the device inoperable after gamma radiation exposure. Advanced transistor design has proposed the use of a boron channel-stop implant to control this parasitic sidewall transistor during pre-radiation. However, under total dose radiation, positive charges (holes) trapped along the sidewall insulator can activate the parasitic sidewall transistor, thereby increasing current leakage or short-circuiting the transistor, leaving associated circuitry inoperable.
Various other techniques have been employed to increase transistor resistance to radiation. The goal of these methods has often been to minimize hole trapping within the insulating region between the transistor gate and semiconductor mesa. In one such structure, a pure oxide sidewall has been shown to yield a device hard to lMRad of total radiation dose. However, the lack of uniformity in such oxide sidewall thickness causes potential points of voltage breakdown between the gate and the underlying semiconductor mesa. Another structure has been proposed which utilizes an oxide/nitride sidewall in an effort to maximize resistance to radiation exposure. The oxide/nitride stack appears to be effective in curing breakdown problems associated with the pure oxide sidewall. However, the operating characteristics of this stack under radiation exposure are complex and are not well understood. Testing to date has shown this stack device to show limited radiation hardness.
Therefore, a need has arisen for a transistor which is predictably hard to high levels of radiation, while minimizing problems in current leakage and voltage breakdown and providing consistent satisfactory operating characteristics.